Rivets are probably the oldest and most widely used fastener in industry. They are inexpensive to manufacture, do not require close tolerance holes, and are easily installed. Riveted structures usually have an extended service (fatigue) life because of the hole-filling characteristics of the rivet. When rivets are driven, the rivet shank expands and tends to fill the hole. This eliminates relative movement between the joined structural members, thus, providing the extended fatigue life.
Another type of fatigue life extending fastener is the interference fit pin. The interference fit is sufficient to put residual hoop tension stresses around the hole which enhances the fatigue life of structures thus joined. These pins are expensive and require very close-tolerance holes and, therefore, are expensive to install. One such fastener is the Hi-Tigue pin, manufactured by the Hi-Shear Corporation, Torrance, Calif.
However, it is more desirable to place residual hoop-compression stresses around the hole. One technique to accomplish this, which has been developed by the Fatigue Technology Corporation, Seattle, Wash., is to expand the hole with a removable sleeve and mandrel. The mandrel, while it is being pulled through the hole, stresses the material around the hole beyond the proportional limit in hoop tension. Thus, when the mandrel is removed, the material around the hole recovers elastically and forms local residual hoop-compression stresses around the hole. Thereafter a fastener is installed in the hole. The additional hole preparation step significantly increases manufacturing costs.
Various rivet designs have been proposed and developed to obtain better control of the filling of the hole when driving. For example, F. S. Briles in U.S. Pat. 3,680,429 discloses a "Self-Gauging, Interference Fit Rivet". The rivet design includes a shank portion having along its length a sequence of progressively reduced diameter sections. When inserting the rivet in a hole, the smallest section freely passes therethrough, while the largest section adjacent to the head requires forced insertion as a result of the interference fit with the hole. In this way the hole is prestressed around the larger section of the rivet. There appears to be no disclosure of expanding the hole to improve the fatigue life.
In the above discussion no attempt has,.been made to make all critical dimensions of the head and tail, before and after driving, of the rivet, a function of the shank diameter. Requiring that the critical dimensions be a ratio of the shank diameter makes the design of any rivet with a particular shank diameter easy to accomplish. While in most cases standard rivet sizes will suffice, there are instances where special diameters (inch-to-metric conversions, etc.) are required. Thus, a rivet having all the critical dimensions based on the shank diameter would minimize design and test time required to insure that the necessary static tension, shear strengths, and fatigue life are achieved.
Applicants' co-pending applications, Ser. No. 657,700, "Controlled Expansion Protruding-Head Rivet Design", filed 10/3/84, Ser. No. 657,300, "Controlled Expansion Flush-Head Rivet Design", filed Oct. 4, 1984 and Ser. No. 676,464, "Controlled Expansion Flush-Head Rivet" filed Nov. 29, 1984, all disclose rivets using both the concept of controlled hole expansion and of having critical dimensions based on the shank diameter.
Additional patents of interest are as follows: U.S. Pat. Nos. 1,480,485, "Rivet" by K. G. Yonce; 3,516,699, "Tapered Rivet" by E. W. Bergere; 1,457,008, "Self Anchoring Rivet" by Smith; 1,966,40 "Rivet" by B. T. Andren, and German Pat. No. 679,962.
Therefore, it is a primary object to provide a rivet that, after driving, obtains controlled expansion of the hole, placing residual hoop-compression stresses in the hole wall, thereby increasing the fatigue life.
It is another object of the subject invention to provide a rivet wherein all critical dimensions thereof are ratios of the shank diameter.
It is another object of the subject invention to retard the formation of the bucked tail to allow more rivet tail material to be pushed into the hole before the bucked tail expansion occurs.
A further object of the subject invention is to provide a rivet that can be installed and driven with or without tank sealant or primer.